I'm 13, and I got my Herbst appliance not too awfully long ago, and I have two cousins stuck with the same curse.
My Herbst appliance rubbed my cheeks raw and bore holes where the four screws are, two on each side of my lips and two at the top. Not to mention, I have a palet expander, which irritated my tongue, giving me sores in 5 areas. I didn't have so much soreness of my teeth or jaw as did the huge, open sores that caused me to have trouble talking, eating, and pretty much everything mouth related.
Now, there is hope! I know the first few days are terrible, I couldn't even swallow drinks because every swallow meant my tongue rubbed against the expander, and it was very difficult to swallow any food because that stupid expander was in the way of any food making its way down. At first, it felt like there was literally no room for solid foods to be in my mouth, and there was way too much spit, like my mouth was a water balloon that never ran out of water.
BUT if you swish salt water (warm water stirred with regular table salt) in your mouth at least twice a day, brush your teeth, and then spray Medline Prevention burn and wound spray on it, you guys, my sores got a lot better. In fact, that's how I survive the first few weeks. Now, I know the prevention spray is for burns and stuff, but it's perfectly safe for your mouth and it helps a lot with the healing and the pain. If it gets really bad, take an ibuprofen.
I also used this thing called a Waterpik Ultra that's like this toothbrush hooked up to a tub of water, expect the toothbrush has all sorts of picks for cleaning in the spots where food gets stuck. It's kinda pricey ($45), but it was worth it for the ease of cleaning.
What else...oh! Dental wax! Yeah, I had no luck with the stuff. I tried the entire two packets full of wax my ortho gave me, and it never worked. I warmed it in my hands, dried the screw area before applying, even stuck it in the microwave, but it always ended up clumping off within a couple seconds and almost gagged on it several times. Not to put you down, it works for some people, but it didn't for me.
I ate lots of soup, especially plain broth when I was learning to swallow and chew, yogurt, pudding, water (obviously), but don't undereat! Eventually, after about the first week, I learned swallow a little better, although it never fully became normal. Still, the sores began to heal up after about two weeks, so don't think that because most people heal after five days that something is wrong, it will get better, I promise. I got mine on two days before Halloween, so I know how much it really sucks when you have about three choices of food, and you're starving but you can't eat a thing.
Just know that it will get better (and I feel bad saying this because everyone says that, right?), they really will, even if it always stinks. I mean, it still takes me an hour to eat a slice of pizza, but I'm only a few weeks in, and my cousin can eat tons of stuff.
Alright, well I've said a mouthful, so I'll get out of your hair. Hope you start to feel better, all you Herbsters!